Graphic arts photography uses light-sensitive photographic materials to record images. The usual product of the photographic process is a negative in which the light portions are represented by dark deposits of silver. When negatives are printed on paper or film, positives are produced in which tone values are similar to the original image. Some printing processes use negatives, others use positives.
Color reproductions of an image can be made by photographing the image through three separate filters. These filters correspond to the three additive primary light colors, red, green, and blue. The three additive primary light colors together form white light.
Placing a green filter over a camera lens produces a negative recording of all green light reflected from the subject. When a positive or print is made from this negative, the silver in the film corresponds to areas that absorbed green light. These areas correspond to the red and blue colors, which are called magenta, a subtractive primary color.
Primary subtractive colors represent the two additive primaries colors left after one primary has been subtracted from white light. There are three primary subtractive colors, cyan, magenta, and yellow. Printing processes usually use inks of the subtractive primary colors to produce color prints. Due to the limitations of the color pigmentations used in inks, a fourth color, black, called key in the industry, is added. Black allows for better contrasts.
High quality printing, such as the quality used for magazines, requires accurate color reproductions. There are several processes that are used to convert from the red, green, and blue primary colors recorded by video cameras, to the cyan, magenta, yellow and key subtractive colors used for the inks in producing color prints.
The accuracy of the color reproduction and the overall quality of the print are also dependant on the accuracy of the registration or alignment of the separate color films. For example, when two films such as those illustrated in FIGS. 1 and 2, are not properly aligned, the resulting print will have a border of a color around the edges between colors, as illustrated in FIG. 3.
Therefore, there is a need for accurately registering the separate color films used to produce a high quality print. An effective way to register the separate color films would be to use a computer. However, there are many difficulties in using a computer to register films such as the computationally intensive nature of the registration process.
Color images can be digitally stored in computers as planes of information. Each plane corresponds to a separate color. Pixels at horizontal and vertical x and y positions have values corresponding to the luminosity of the color at that positions wherein a "0" indicates no amount of the color is present while a "1" indicates the greatest possible amount of the color is present.
There are some video cameras which do not have registration errors because only one sensor is used for the image and the image data is divided into separate planes after the image as a whole is recorded. It is not always desirable to use these video cameras because of problems with resolution, and color fidelity.
Scans of contone, or continuous tone, color images using a high quality drum scanner do not have registration errors. However, scans of halftone images, containing very small dots of varying sizes used in many type of printing processes, can have registration errors.
Some registration methods, require that registration be accurate during scanning. However, multisensor video cameras can have errors in registration.
Even when a print is made from separate films that were formerly registered, the registration can become inaccurate due to the stretching of the film and other problems with mechanical tolerances.
Complex warps, as shown in FIGS. 5 through 7, such as parabolic warps, can occur in films that have been stored for long periods of time prior to making a print. Complex warps also can arise during digital preliminary. Digital preliminary digitally captures and processes images without the need for film.
There is a problem in efficiently and accurately registering films to imperceptible levels especially when image planes are warped in complex ways. There is also a problem in being able to register planes with or without test images.
The present invention is directed toward overcoming one or more of the above problems.